1. Field of the Invention
The present invention relates to an organic transistor, a method of forming an organic transistor, and an organic EL display using an organic transistor. The organic semiconductor film for organic transistors of the present invention is manufactured by printing processes such as inkjet printing process and relief printing process.
2. Description of the Related Art
In recent years, there has been a progressive reduction in the price of thin displays, such as plasma displays and liquid crystal displays, and consequently, there is an urgent need to produce such thin displays at lower cost. Accordingly, as low-cost production techniques, printing processes such as inkjet printing process and relief printing process are drawing attention. For example, semiconductor films for silicon transistors, which currently are mainly used, are formed by vacuum process (vacuum deposition, sputtering, etc.) and photolithographic process, and there is a problem that the manufacturing cost is very high. On the other hand, organic transistors with organic semiconductor films formed by printing process using organic semiconductor material can be manufactured at very low cost.
To form an organic semiconductor film by printing process using an organic semiconductor material, ribs (also referred to as “banks” etc.) are required to enclose the regions where the organic semiconductor film is positioned. A rib refers to a physical boundary formed of organic material (polymer, monomer, etc.) or inorganic material.
For example, Japanese Patent Application Laid-Open No. 2003-318131 discloses a method of making device by forming a rib in the perimeter part of an region where a liquid material is to be applied, applying the liquid material to the region enclosed in the rib and drying the liquid material. This method makes vacuum process unnecessary, and, by enclosing the region where liquid material is applied with a rib, the amount of liquid material to be used can be reduced to a minimum, thereby saving the manufacturing cost.
On the other hand, Japanese Patent Application Laid-Open No. 2006-237477 discloses a method of forming a gate electrode by a coating method using a resist mask for a rib, wherein the resist mask also had been used for etching of a gate insulating film.
To make device by printing process as described above, it is necessary to form a rib. Forming a rib generally requires, for example, the following steps of: deposition of a photosensitive resin film by spin coating→baking→exposure→development→cleaning. This formation of a rib is one factor of increased cost of device making.
To form a liquid material in a cell enclosed in ribs by printing process, there is a need to provide a difference in the lyophilic property (or the liquid repellency) between the rib surface and the cell surface. To be more specific, the rib surface needs to be controlled to be water repellent and the cell surface needs to be controlled to be hydrophilic. Methods for such control include making surface lyophilic by oxygen plasma process and making surface liquid repellent by CF4 (tetrafluoromethane) plasma process. However, there is a problem that liquid repellency or hydrophilicity obtained by plasma process is maintained only for several tens of minutes and disappears in a short time. This makes stable device production difficult.
In addition, in the event that the substrate region is large or the material for the substrate is a flexible material such as plastic, there is a problem that, when a rib is formed by photolithography, alignment with the base becomes less accurate and therefore the product yield decreases.
Furthermore, when an organic transistor is made by a coating method using a rib formed by a conventional method, given that the alignment accuracy with the exposure device in the photolithography step is limited, the source/drain electrode and the gate electrode may overlap each other more than necessary. When the source/drain electrode and the gate electrode overlap each other too much, the parasitic capacitance increases, which may cause degradation in transistor characteristics.